An optical waveguide Mach-Zehnder interferometer has a structure in which an optical waveguide is branched and one or both of branched optical waveguides are applied with an electric field parallel to a crystal axis thereof to phase-shift light beams propagating therein, which beams are thereafter combined again. Because a light intensity after combined is varied by the electric field applied thereto, the interferometer is used as an electric field sensor for detecting, by measurement of the light intensity, an electric field intensity applied to antennas connected to electrodes. The intensity of an outgoing light beam of the Mach-Zehnder interferometer exhibits a trigonometric function wave curve with respect to the electric field applied to the electrodes.
FIG. 1(a) shows one example of a conventional optical electric field sensor. As illustrated in the figure, the optical electric field sensor comprises an optical branched waveguide type interferometer formed on an LiNbO.sub.3 substrate by diffusion of Ti. One of two branched optical waveguides is provided with electrodes to form an optical modulator. The optical modulator is fixedly housed in a case 1 made of plastic. The electrodes of the optical modulator are connected to antennas 2, respectively. A polarization maintaining fiber 3 and a single mode fiber 4 are connected to a light incident side and a light outgoing side of the optical modulator, respectively. Connectors 6 are provided at the ends of fibers 3 and 4. An electric field spontaneously or forcedly generated is transmitted through the antennas to the electrodes to produce phase modulation in the optical waveguide. The light beam combined thereafter is modulated in intensity and, thus, has the light intensity corresponding to the electric field.
FIGS. 2(a)-2(d) show a conventional optical waveguide Mach-Zehnder interferometer used in the optical modulator illustrated in FIG. 1(a). As illustrated in FIG. 2(a), the optical waveguide Mach-Zehnder interferometer has a structure such that an optical waveguide is branched into branched optical waveguides 12 and 12 arranged on substrate 21, one or both of which are applied with an electric field 18 parallel to an optical axis through modulation electrodes 22 and 22 to provide phase-shift in the optical waveguides before being combined again. An input light beam is shown at 15 in FIG. 2(a), and an output light beam is shown at 16. Because a light intensity after combination is varied by the electric voltage applied thereto, the interferometer can be used as an electric field sensor for detecting, by measurement of the light intensity, an electric field intensity applied to antennas 2 as a low voltage applied across the modulation electrodes 22 and 22.
FIG. 3 shows an optical modulation characteristic of the Mach-Zehnder interferometer illustrated in FIG. 2(a). As illustrated in FIG. 3, an output intensity (relative intensity) of the light beam modulated in intensity by the Mach-Zehnder interferometer varies along a trigonometric function wave (sine wave) curve with respect to the applied voltage. In view of the above, adjustment (optical bias adjustment) is performed so that the light intensity is located at a linear variation point (a middle point between the maximum level and the minimum level) of the trigonometric function wave when the applied voltage is equal to 0 V. In this event, variation in light intensity and the applied electric field exhibit a proportional relationship. It is therefore possible, as an electric field sensor, to measure the applied electric field by the light intensity. In other words, such a characteristic is required for use as an electric field sensor.
The conventional optical electric field sensor, however, has a distance between the electrodes which is as small as several microns. If foreign substances, such as alkali ions, exist between the electrodes, the voltage applied across the electrodes is accumulated as a charged voltage. This results in fluctuation of an optical modulation ratio with respect to the applied voltage. Such fluctuation tends to occur in a low frequency rather than in a high frequency (DC drift, giving a largest influence upon a direct-current voltage). In that event, measurement accuracy of the optical electric field sensor is deteriorated. When the optical electric field sensor of this type is subjected to temperature variation, carrier particles are generated within a crystal, moved, and nonuniformly accumulated in the vicinity of the electrodes to produce an internal electric field. This results in instability (temperature drift) of the outgoing light beam. Such fluctuation in characteristic is great and small when the temperature variation is drastic and gentle, respectively. The temperature drift will briefly be described in conjunction with FIG. 1(b) and FIG. 1(c). Referring to FIG. 1(b), the optical electric field sensor is put in a condition where an ambient temperature is equal to 30.degree. C. which is higher than a room temperature. An incident light beam is incident to the polarization maintaining fiber 3 (FIG. 1(a)) and passes through the conventional optical electric field sensor to be emitted from the single mode fiber 4 as a normal output light beam having a waveform A. An abscissa and an ordinate represent an applied electric field and a light intensity, respectively. Herein, adjustment is made so that the light intensity is located at a middle point between the maximum level and the minimum level when the electric field applied to the antennas is equal to 0 (V). As far as a normal operation is carried out, the waveform is as illustrated in FIG. 1(b). When subjected to the temperature drift, the output light beam emitted from the single mode fiber 4 has a waveform B illustrated in FIG. 1(c). In the waveform B, the light intensity is phase-shifted by .pi./4 with respect to the waveform A of the incident light beam when the electric field applied to the antennas is equal to 0 (V). Such shift is the temperature drift which deteriorates the temperature characteristic of the optical electric field sensor. As a result, the sensitivity becomes unstable.
In order to improve the temperature characteristic, the optical modulator used in the conventional optical electric field sensor adopts a method of indirect compensation. Specifically, the optical crystal is given distortion equal in magnitude and reverse in polarity to the drift by, for example, application of a physical stress caused by a Peltier element or the like, and alternatively, addition of an extra electric field reverse to the distortion the modulation electric field. As known in the art, such fluctuation in characteristic can be avoided by forming a conductive film on the surface of the substrate to cancel the electric charge within the crystal.
However, there has been no such optical electric field sensor that has a structure for suppressing heat conduction to the optical modulator, which heat conduction substantially is a cause of deterioration of the temperature characteristic. In order to monitor the output of the optical modulator, to measure the temperature drift, and to apply distortion for compensating it as described above, a device for operating these mechanisms is required. Furthermore, an accuracy is required. In addition, a typical optical modulator uses the conductive film such as a semiconductor Si film to suppress the fluctuation in characteristic. However, because sputtering or vacuum deposition is adopted therefor, there arises a problem of an increase in process time.
On the other hand, when the above-mentioned Mach-Zehnder interferometer is manufactured, the optical modulation characteristic with respect to the applied voltage generally changes in dependence upon the characteristic of the LiNbO.sub.3 substrate or the manufacturing condition of the element. Specifically, it is possible to assure a reproduciability of those characteristics such as a half-wavelength voltage and a loss. However, it is difficult to adjust the light intensity at the applied voltage of 0 V to the middle point between the maximum level and the minimum level as required to the electric field sensor. In view of the above, it is a general practice to carry out adjustment (optical bias adjustment) by giving distortion to the waveguide after manufactured.
In the meanwhile, the electric field sensor has a structure in which the antennas made of metal receive the electric field to generate the applied voltage at the electrode portions of the optical modulator. When any metal other than the antenna is present around the sensor, the electric field generated around the electric field sensor is disturbed. Therefore, the package is preferably made of a nonmetallic material to remove metal components other than the antennas. Use is generally made of resin such as plastic. The electric field sensor thus manufactured is used to measure the electric field intensity on the order of several mV/m because of its characteristic, and is readily subjected to the influence of the electric field generated therearound. In addition, the package made of resin such as plastic generates an electrostatic field having such a level that fluctuates the optical bias. Since the electrostatic field generated by the package is greatly concerned with variation of humidity or the like, it is difficult to provide an element having a constant optical bias. However, in order to compensate for deviation of the optical bias due to the electrostatic field, consideration has mainly been directed to adjustment of the optical bias after packaging.
It is therefore one object of this invention to remove an electrostatic field generated by a package material after packaging as well as to improve heat insulation of an optical waveguide element so as to remove fluctuation of an optical bias due to temperature drift of a Mach-Zehnder interferometer.
It is another object of this invention to provide an optical electric field sensor which has a structure for suppressing heat conduction of thermal fluctuation outside of the optical electric field sensor to an optical crystal, to thereby dispense with the device requiring the high accuracy and to improve a temperature characteristic.
It is still another object of this invention to provide an optical electric field sensor having a conductive film formed by an inexpensive and simple process.
It is other object of this invention to provide an optical electric field sensor which is capable of inhibiting interference with an external environment to readily prevent invasion of dirt or foreign substances by applying an agent having a stable characteristic on an area between electrodes where invasion of the foreign substances is otherwise easy.
It is a further object of this invention to provide an optical electric field sensor having a structure of removing an electrostatic field generated by a package material after packaging to thereby avoid disturbance of an electric field to be measured.